1. Field of the Invention
The invention relates to a method of generating modulator output signal values when employing a multi-level modulation method, i.e. a modulation method whose signal space diagram has several points depicting a state or level.
2. Description of the Related Art
In digital communications, for instance radio telecommunications systems or optical fibers, the signal being transmitted is typically a carrier signal modulated with information bits. Modulation makes it possible to transfer several signals in one and the same channel on different frequencies. Modulation can be both digital and analogue. Digital modulation methods can be divided into three basic methods which can also be combined: ASK or amplitude shift keying, PSK or phase shift keying, and FSK or frequency shift keying. The amplitude of the carrier is altered in amplitude shift keying, the phase of the carrier is altered in phase shift keying and the frequency of the carrier is altered in frequency shift keying, according to the information bits. In most cases, present radio telecommunications systems use different phase shift keying methods, which include MSK or minimum shift keying, QPSK or quadrature phase shift keying, and OQPSK or offset quadrature phase shift keying.
The modulated signal is often depicted using a signal space diagram. FIG. 1 shows an example of a two-dimensional signal space diagram of a phase-modulated signal, when the modulation has four levels. The system then uses four different signals or pulse formats. In the example of FIG. 1, points 100, 102, 104, 106 depict different signals, i.e. states of the signal space diagram. In the signal space diagram, the pointer diagrams of the signals have been combined in one figure. FIG. 1 shows a pointer diagram arrow 108 which depicts the amplitude of one signal, and an angle 110 which depicts the phase. For the signal, the pointer diagram shown is A cos(2πf0t+φ), where A is the signal amplitude, f0 medium frequency, t time, and φ phase. In the different states 100, 102, 104, 106 of the signal space diagram, the signal obtains a different phase. The number of states in a signal space diagram varies depending on the modulation method; the more states, the higher the data transmission capability the system has. A signal space diagram can be shown as a unit circle as in FIG. 1, but other presentation methods exist. The example of FIG. 1 is a signal space diagram of quadrature signals, in which the axes are Q representing a quadrature component and I representing an in-phase component.
Today, a need has arisen to increase the data transmission capacity of radio telecommunications systems, such as cellular radio systems, to make a wireless use of the Internet or video telephone calls possible. One solution to this problem is using multiple-level modulation. In the EDGE (enhanced data rates for GSM evolution) system, for instance, the data transmission capacity of the GSM (Global System for Mobile Communications) system is improved by shifting from the MSK modulation to 8-PSK, i.e. an eight-level, phase modulation. There is, however, the problem of implementing a multi-level modulation sufficiently quickly and efficiently.